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NNTP-Posting-Date: Mon, 05 Mar 2007 23:33:31 -0600
From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Does Ada tasking profit from multi-core cpus?
References: <rgcukvs1j6ck$.1vjj69zzy1h56$.dlg@40tude.net>
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Date: Mon, 05 Mar 2007 23:33:31 -0600
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Date: 2007-03-05T23:33:31-06:00
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> 1. Whether protected object's functions are indeed executed concurrently
> when come from the tasks running on different cores?

   A quick test with a single protected object containing a single,
long-duration, function appears to have just one call of the function
active at a time, even if the function is called from two different tasks.

  global_flag : integer := 0;

  protected body pt is
    function f(id : integer) return natural is
      change_count : natural := 0;
    begin
      global_flag := id;
      for i in 1 .. 10_000_000 loop
        if global_flag /= id then
          change_count := change_count;
          global_flag := id;
        end if;
      end loop;
      return change_count;
    end f;
  end pt;

One task calls pt.f(id=>1) and the other calls pt.f(id=>2).  They both get
a result of zero back from their function call.  This was with Gnat 3.15p
Windows 2000 on a dual core Pentium.  If I change it from a single
protected object to two instances of a protected type, then the function
calls are overlapped and return non-zero results.

> 3. Can a task switch cores? If yes, what is the overhead of switching?
  By "switch cores" do you mean that the particular hardware stack
pointers swap which stack they are pointing to?  I think this is an OS
question and, for Windows, I don't know how one asks "which core am
I currently running on" - or indeed if that questions makes any sense.